A tensioning device with the aforementioned characteristics is described in EP 1 283 138 A1; the known pre-tensioning device consists of a tube which is bent for better packaging and assembly in the often limited space available in a motor vehicle for the accommodation of a drive which is necessary for the pre-tensioning process, which forms a straight section which accommodates a piston and therefore forms a piston path section, and a tube section which is offset in relation to the piston path which accommodates the gas generator. In the case of the embodiment shown in EP 1 283 138 A1, the pre-tensioning device is provided for a belt buckle, whereby the belt buckle is attached to a cable. The cable section is guided in an additionally provided cable guide area outside the belt tube, guided into the tube by means of a cable opening formed in the walls of the belt tube and is connected to the piston. Upon triggering, the gas generator generates a drive gas which streams through the bent area of the tube and drives the piston along the piston path, so that the cable connected to the piston is pulled into the bent tube. By these means, the belt buckle is pulled in the direction of the cable guide, tightening the seat belt connected to the belt buckle.
The known pre-tensioning device is associated with the disadvantage that the pressure of the gas generated by the gas generator is led directly to the piston through the section of the tube which has a constant cross-sectional shape. This means that the pressure peak which is created after ignition of the gas generator and before the movement of the piston necessitates sufficient strength of the drive cable and the connection of the cable with the piston as well as a correspondingly good seal of the cable entry into the tube, which gives rise to corresponding costs and effort when designing the known pre-tensioning device.
Therefore this invention is based on the task of designing a pre-tensioning device with the above-described generic characteristics in such a way that the complexity of loading of the drive cable and further components is reduced, while the pre-tensioning device is manufactured in a simple way.